Silicon wafers, sometimes as large as eight inches in diameter, are used in the manufacture of integrated circuits (hereinafter "ICs"). A typical silicon wafer contains dozens, sometimes hundreds of individual ICs, depending on the size of the IC being fabricated. Following the fabrication process, the individual ICs are separated from the wafer by sawing, to form IC chips or dies. Each IC chip or die undergoes a packaging process in which the die is mounted and attached to a leadframe with an epoxy or other suitable adhesive. The leadframe usually includes a patterned strip of electrical conductors. Electrical connections are made between each IC and the associated leadframe. The entire structure is typically encapsulated within a polymeric material to protect the IC and electrical contacts from the environment. This packaging process produces products known as plastic electronic packages.
Since the epoxy or other suitable adhesive (hereinafter "die attach") is typically a thermal set material, the curing process requires that a minimum temperature be obtained before it will completely solidify. Overheating the die attach causes the die attach to become brittle resulting in cracks and degraded performance. Under cured die attach will often result in poor adhesion between the die and the associated leadframe causing the die to separate from the leadframe during subsequent process steps. Hence, it is very important that the temperature of the curing oven be monitored and controlled.
Many ICs are also placed into ceramic packages which provide better protection against the environment. This assembly process is similar to plastic electronic packaging, except different materials and processing temperatures are used. For example, the die attach for ceramic packages often includes a silver/glass adhesive. Silver and glass particles are held in suspension by various organic solvents. The silver/glass die attach has the consistency of a paste and is not cured in the same manner as epoxy compounds, but is thermally processed to "burn off" the organic solvents so the silver/glass mixture will form a solid, high quality bond.
The temperature control for the die attach curing process is typically accomplished by monitoring and adjusting the ambient temperature inside the oven. The oven temperature is typically measured by suspending one or more thermocouples inside the oven as near the parts being processed as possible. For certain processes, indirect temperature measurement of the oven is satisfactory if the program temperature changes are stretched over an extended period of time allowing the part to heat up or cool down at a relatively slow rate. For these processes, the temperature of the parts will be nearly the same as the oven temperature.
Recently, new die attach materials have been introduced in the electronic packaging industry which require novel approaches to monitoring and controlling the temperature of the part. The new die attach materials rapidly cure in a matter of one to five minutes as compared to a more standard cure cycle of thirty to sixty minutes.
One major advantage of these rapid cure processes is the dramatic reduction in cycle time and operating cost. Nevertheless, these rapid cure processes represent a new set of design rules for the accurate and repeatable control of the cure temperatures. For many of these new designs, it is no longer possible to correlate the ambient temperature of the oven to the temperature of the die being processed because the temperature of the die is changing much faster than the ambient oven temperature. The chances of dies either being under or overheated are more likely to occur.